This invention relates to navigational receivers in general, and in particular, to an avionics system for receiving OMNI mode VOR signals, ILS localizer signals (LOC) and ILS glide slope signals (GS) on a time-shared multiplexed basis.
Instrument landing systems (ILS) are utilized as navigational aids to landing. ILS includes three basic ground station signals: (1) localizer signals (LOC): (2) glide slope signals (GS); and (3) marker signals. The LOC defines a vertical plane of radiation created by generated signals which permit an aircraft to align with the centerline of a runway. A received amplitude balance establishes this centerline. GS defines the proper angle from an initial approach plane to the plane of the runway which will enable the aircraft to make a proper landing, typically an angle of 3.degree.. Equal 90 Hz and 150 Hz amplitude modulation components provide an indication the aircraft is at the desired slope angle of 3.0.degree.. The 150 Hz output predominates below the glide slope while the 90 Hz output predominates above the glide slope. An amplitude comparator determines whether the aircraft is below or above the desired glide path. Marker signals are simple 75 MHz narrow profile signals, vertically, projected, which intercept an aircraft passing overhead, indicating position.
Most commonly, a transmitter identification signal is superimposed on the LOC signal. This identification signal is in Morse Code to give the identification code in "dots" and "dashes" for a ground station.
In the past, separate and distinct avionics systems were developed to process each separate signal within the aircraft navigational system. As such, VOR/LOC signals are received exclusively by a dedicated antenna and handled exclusively through a separate RF amplifier, RF mixer, IF filter, IF amplifier, detector, buffer, converter and display. Similarly, GS signals were received exclusively by a dedicated antenna and handled exclusively through another separate RF amplifier, RF mixer, IF filter, IF amplifier, detector, buffer, converter and display.
As there is a duplication of components such as IF filter, IF amplifier, detector, buffer and tone filters when separate and distinct systems are used, it is possible to time-share process VOR/LOC and GS signals through commonly dedicated IF filter, IF amplifier, detector, buffer and tone filter (intermediate components) on a multiplexed operational basis. Such a multiplex receiver system, could utilize a multiplex control circuit to control the operation of the navigational receiver dual front end for multiplexing signals coming from a separate VOR/LOC front end and a separate GS front end into the shared intermediate components and controlling the operation of dual output display circuits, demultiplexing signals coming through the intermediate components into the exclusively dedicated output display circuits.
Multiplex processing in such a system would occur regardless of the desires of the pilot/navigators. It is also, however, desirable to verify the identity of a ground station, this being done by the pilot who must listen to the Morse Code signal which has been superimposed upon the LOC signal. As all Morse, "dots" and "dashes" are transmitted at a frequency of 1020 Hz for periods of 100 MS and 300 MS, respectively, this tone frequency will be interrupted due to the time-shared multiplex operation of the receiver, thus, rendering it unrecognizable to the pilot.
An objective of this invention is to provide a keyed identification tone extraction scheme for use in a multiplex operated navigational receiver.
Another objective of this invention is to provide a scheme for reconstituting an identification tone extracted whereby discontinuities in the tone signal are filled in.